Get all invalid from a list LINQ

Question

I have two lists that I have a property that I want to compare on. Based on this property, I want to return all the elements from list1 that is not in list2.

List1: {Name, Age, Id}

{John, 10, 1},
{Joe, 20, 2},
{Mary, 30, 3}

List2: {Name, Age, Id}

{John, 10, 1}
{Joe, 20, 2}
{Mary, 30, 5}

This should only return from list 1

{Mary, 30, 3}

I've tried

var invalidElements = list1
                .Select(o => list2.Any(p => p.Id != o.Id))
                .ToList();

Does this answer your question? [Linq find differences in two lists](https://stackoverflow.com/questions/2404301/linq-find-differences-in-two-lists) — Jim G., Aug 19 '22 at 17:45
Does this answer your question? "[Compare two list of objects and select common and difference records based some property](/q/9926091/90527)", "[compare properties in classes of list in class](/q/18742293/90527)", "[C# Find difference of two custom object lists based only on some properties](/q/44554700/90527)" — outis, Aug 20 '22 at 18:26
… "[Get the differences between 2 lists](/q/10810438/90527)", "[LINQ expression to get difference from 2 list with a property](/q/31828664/90527)", "[Comparing 2 lists and return difference in a 3rd list](/q/33752401/90527)" — outis, Aug 20 '22 at 18:31
As per the [site guidelines](/help/how-to-ask) in the [help], please [search](/help/searching) before posting. See also "[How much research effort is expected of Stack Overflow users?](https://meta.stackoverflow.com/q/261592/90527)" — outis, Aug 20 '22 at 18:36

Luke · Accepted Answer · 2022-08-19T17:55:53.910

2

Use the Where method instead of select (and invert your logic). Select is used to project your collection to another of a different type (often a property of your element).

var invalidElements = list1
                .Where(o => !list2.Any(p => p.Id == o.Id))
                .ToList();

edited Aug 19 '22 at 17:55

answered Aug 19 '22 at 17:31

Luke

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score 1 · Answer 2 · answered Aug 19 '22 at 18:00

1

Because you wrote that you want to filter by Id, you can use the new ExceptBy method:

var invalidElements = list1.ExceptBy(list2.Select(a => a.Id), x => x.Id);

answered Aug 19 '22 at 18:00

A.B.

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Get all invalid from a list LINQ

2 Answers2